1. Field
Disclosed is a method and apparatus of simulating the optical properties of one or more intraocular lenses following implantation under real-world conditions at far away, close, and intermediate distances, and under monocular or binocular viewing conditions.
2. Description of the Related Art
Some fundamental optical concepts concerning presbyopia, cataracts, and cataract extraction and implantation of intraocular lenses are provided in order to facilitate an understanding of the method and apparatus.
The convergence, or divergence, of light rays from an object is measured in optical units known as Diopters “D” where D=1/F, and F is the distance between the object and its image in Meters. By convention, converging light is designated by a positive sign (+D) and diverging light is designated by a negative sign (−D).
A normal eye has a converging optical power at rest of about +60D and it focuses distance objects to a clear focus on the retina. About two-thirds of the eye's optical power is provided by the corneal surface and the remainder of the optical power is provided by the crystalline lens inside the eye.
The image of an object 6M away has a diverging power of ⅙ D, a value which is insignificant relative to the total optical power of the eye. Clinicians skilled in the art consider objects at 6M, or beyond, to have optical properties that are similar to objects located at infinite distances.
An object located 1 meter from the eye produces light rays that have a divergence of −1 D and an object at 0.25M, a typical reading distance, produces light rays with −4D of divergence. The divergence of light rays from these nearby objects must be neutralized by the eye in order to focus the image of the near object sharply on the retina where it can be perceived clearly.
In youth, the additional optical power needed is provided by the crystalline lens inside the eye which becomes more spherical and more optically powerful when attention is focused on near objects. The triad of actions that occur when viewing a near object consists of convergence, or nasal deviation, of the optical axes of the eyes; miosis—a decrease in the diameter of the pupils; and accommodation—an increase in the optical power of the crystalline lens thereby providing the additional focusing power needed for viewing near objects clearly.
Presbyopia is the age-related loss of accommodation, which requires middle-aged patients to wear reading glasses or bifocals to see near objects clearly. Patients who have had their crystalline lens removed due to cataract, trauma, or other disease are, in effect, presbyopic, and they must wear optical correction for clear near vision.
A cataract is a clouding of the eye's clear crystalline lens that impairs vision. Cataracts are most commonly caused by aging processes in the lens. Once the leading cause of blindness, cataracts are now treated effectively by removal of the opaque natural lens, or cataract, and by the implantation of an intraocular lens, or “IOL,” to restore vision. In the United States, cataract surgery is the most commonly performed surgical procedure with about 3.5 M procedures performed in 2010.
Initially, patients who had cataracts removed had to wear very powerful, thick spectacle lenses to restore the optical power of the crystalline lens that was removed. These “aphakic spectacles” caused distortion of objects and caused the patient's eyes to appear abnormally large to others. However, without their thick aphakic spectacles, these patients were functionally blind.
Subsequent development of contact lenses with high plus power to restore the power of the crystalline lens that was removed (aphakic contact lenses) provided patients with improved optical performance and cosmetic appearance following cataract extraction.
In WWII, Harold Ridley, a British surgeon, observed that plastic shards that penetrated the eyes of aviators were well tolerated for long periods of time. Based upon his observations, Ridley devised a plastic intraocular lens replacement to restore the optical function of the crystalline lens that was removed during cataract surgery. Ridley's work spawned the evolution of modern intraocular lens implants (IOLs) which have undergone significant improvements in optical designs and materials during the last 50 years.
Today, modern cataract surgery with the implantation of an intraocular lens using soft acrylic, silicone, hydrogel, and other biocompatible polymeric materials is commonly performed and considered to be the standard of medical practice in the United States and developed countries around the world.
The first intraocular lenses had a single optical power that was generally selected to provide the patient with good distance vision following surgery. For intermediate and near distances, reading glasses such as bifocals, trifocals, or multi-focal progressive add spectacle lenses (PALs) were prescribed following surgery.
Advancements in lens manufacturing techniques and in the understanding of physiological optics permitted the development of more complex IOL designs for the correction of astigmatism. More recently, presbyopia-correcting IOLs have been developed. The purpose of presbyopia correcting IOLs is to provide the post cataract surgery patient with satisfactory distance, intermediate, and near vision that permits them to conduct their daily activities without glasses.
Presbyopia Correcting IOLs (PC-IOLs).
Presbyopia correcting IOLs of refractive, diffractive, and accommodative designs are known.
The refractive type of presbyopia correcting intraocular lens has a lens surface that is composed of different surface areas, each with different radii of curvature with transitional regions that have aspherical curvatures. The diffractive type of presbyopia correcting IOL employs a diffractive, or Fresnel lens array, an optical design that features alternating rings of lenslets, each possessing a different optical power. Presbyopia correcting IOLs of accommodating designs are designed to move along the optical axis of the eye in response to changes in the ciliary muscle that occur when the patient views a near object, thereby increasing the effective optical power of the IOL.
Each type of presbyopia correcting IOL has advantages and disadvantages. Diffractive IOLs have predictable near add power, but they scatter approximately 20% of the light and degrade image quality to a degree that some patients find unacceptable. Presbyopia-correcting IOLs of refractive designs may be particularly sensitive to the size of the pupil and to decentration and tilt of the IOL. Accommodative lenses may not always couple to the delicate intraocular structures and move in the way their designers intended.
Within each type of presbyopia correcting IOL, there are numerous lenses with variations in design that reflect the philosophy and experience of the lens manufacturer. Examples of presbyopia correcting IOLs may be found in U.S. Pat. Nos. 7,717,558 and 6,685,741.
Despite the potential for presbyopia correcting IOLs to free post-cataract surgery patients from the need to wear corrective eyewear after surgery, there are major unsolved problems with prior art methods for selecting appropriate candidates for PC-IOL surgery, for identifying patients who will be dissatisfied with their implant, for determining which lens design will work best for a given patient, and for improving the designs of PC-IOLs.
In most instances, the patient must pay an out of pocket fee for a presbyopia correcting IOL in addition to what is covered by Medicare and private insurance programs. There are presently limited and ineffective tools available to demonstrate to the patient the benefit that a presbyopia correcting IOL will provide in comparison to the vision that a monofocal IOL provides. Thus, patients are presently asked to pay a considerable fee for a benefit that cannot be realistically demonstrated to them before surgery.
As more and more presbyopia correcting IOLs become available, ophthalmologists and their patients are exposed to an expanding number of choices and marketing claims that are difficult to evaluate objectively. Unlike the shopper who may try on a new shoe to assess its fit and comfort before purchase, patients cannot presently preview and compare the vision that will be provided by different IOL designs prior to their cataract-IOL surgery.
Clinical experience with PC-IOLs has demonstrated that not all patients are good candidates for these lenses. Some patients are so dissatisfied with their vision following surgery that the PC-IOL must be explanted and replaced by a lens of different design. A lens exchange procedure subjects the patient to the additional risks and costs of surgery.
It has been reported that patients who are highly detail-oriented and/or who have demanding personalities are poor candidates for PC-IOL surgery, yet there are no commonly accepted objective standards for making such an assessment. Some surgeons believe that patients who have undergone prior corneal refractive surgery such as radial keratotomy (RK) or laser vision surgery (PRK or LASIK) should not receive any type of presbyopia correcting IOL. Yet, other surgeons favor the implantation of a particular presbyopia correcting IOL for these patients. Some surgeons recommend that presbyopia correcting lenses of different design be implanted in each eye, a so-called mix and match approach. Yet, other surgeons dissuade such a strategy.
Confounding the lack of consensus on standards for selecting presbyopia correcting IOLs is the fact that patients have different lifestyles and different visual needs and the different IOL designs vary in their ability to meet these visual needs following surgery.
Measurement of Intra-Ocular Lenses.
Methods to measure the optical properties of IOLs are known. U.S. Pat. No. 7,944,533 to Alcon, Inc. teaches a method of measuring a multifocal diffractive IOL by using a Hartmann Schack wavefront sensor that has a sufficiently dense array of lenslets to measure the optical properties of the IOL.
Spatially resolved refractometers are known, such as the device described by Webb in U.S. Pat. No. 6,000,800. A spatially resolved refractometer may be modified to determine the optical characteristics of a 6 mm diameter IOL by introducing an aperture that is small relative to the optical component measured. For example, a diffractive IOL may have rings of 0.25 mm in width. Therefore an appropriate aperture size of 0.1 mm may be selected to scan and characterize the optical properties of the IOL.
Wave Plates.
Wave plates that impart a phase change to the wavefront of an image are known. A suitable wave plate may be manufactured from PMMA or other optically suitable material using laser fabrication means or by computer controlled lathe means.
Means to emulate the outcome of a surgical procedure are known by those skilled in the art. For example, monovision can be emulated by trial of contact lenses that approximate the outcome that can be expected following laser vision surgery. Wave plates to simulate outcomes of laser vision procedures are known as well.
IOL Simulators.
Multifocal IOL simulators are known such as the device described in US Patent application US 2011/0080562. This application teaches a multifocal intraocular lens simulator that includes an optical system that allows an object to be observed through it, and a test lens holder which holds a prescribed test intraocular lens. The intraocular lens holder is installed at a position optically conjugate with a position at which an eye of an observer is to be placed.
The prior art method of simulating a multifocal lens does not provide the patient a realistic assessment of the quality of vision that the multi-focal IOL will provide, because no means are provided to view objects of varying size, shape, color, contrast, and illumination; nor does the aforementioned patent application teach a means to view objects at near, intermediate, and far away distances.
Prior art devices provide no clinically practical method for determining which, if any, of the available designs of presbyopia correcting IOLs will provide a given patient with a satisfactory level of visual function following implantation of an intraocular lens, nor do they permit the patient to preview, compare, and select the IOL design that they prefer based upon a comparison of the IOLs image producing properties.
Unsolved Problems.
As a result of these unsolved problems, the market adoption of PC-IOLs has been far below predicted values as many patients and surgeons have low confidence in the patient's satisfaction with the vision provided by the PC-IOL. The inability to assess the performance of different lens designs without implantation has slowed the innovation improved designs.
To address these unsolved problems, this disclosure teaches a new method and apparatus that permits patients to preview and compare the distant, intermediate, and near vision that a particular IOL lens design will provide, and allow the patient to compare the vision provided by a plurality of designs while observing realistic images of real-world scenes over a variety of viewing distances. This provides patients, before surgery, the ability to preview, compare, and select the IOL lens design that is most likely to provide satisfactory visual function following surgery.